Electric power steering device and resin gear used for the same

ABSTRACT

An electric power steering device having a resin gear used in a speed reduction gear mechanism and the resin gear used for the device are provided. The speed reduction gear mechanism includes a driven gear that is the resin gear integrally formed by fitting a resin part having gear teeth formed on the outer peripheral surface thereof to the outside of a metal core and a drive gear meshed with the driven gear. The resin part of the resin gear is formed of a resin composition having, as a base resin, a polyamide resin containing 10 to 50% by weight of glass fiber of 5 to 9 μm in diameter, and has excellent wear resistance, durability, and dimensional stability. Grease on the meshed surfaces of the gears has such a composition that includes a thickener and 3 to 10% by weight of a wax having a melting point or softening point in the range of 70 to 130° C. in a base oil formed mainly of at least one oil selected from mineral oil, poly α-olefin oil, and alkyl polyphenyl ether.

RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/JP2004/003629, the filing date thereof being Mar. 18, 2004 and thepriority date being Mar. 19, 2003, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an electric power steering device, andparticularly to an electric power steering device having a resin gearused in a speed reduction gear mechanism constituting a power assistportion and a resin gear used for the device that is suitable for powertransmission.

BACKGROUND ART

In an electric power steering device for vehicles, a speed reductiongear mechanism is installed between an electric motor and a steeringshaft because the electric motor having relatively high revolutions andlow torque is used. Speed reduction gear mechanisms with the use of aspur gear and other gears are known as the speed reduction gearmechanism. However, the use of a well-known worm speed reduction gearmechanism formed of a worm and a worm wheel is more typical because alarger speed reduction ratio can be obtained by a set of them, and soforth.

Such a worm speed reduction gear mechanism (hereinafter, simply referredto as reduction gear) is constructed from the worm that is a drive gearconnected to a rotating shaft of an electric motor and a worm wheelmeshed with the worm.

When both of the worm and the worm wheel in such a reduction gear aremade of metal, there is a disadvantage that uncomfortable sounds such asrattling sound and vibration sound are generated when the steering wheelis handled. When the worm is made of metal, this problem has beenconventionally solved by using a worm wheel provided with and formedintegrally with a teeth portion made of a synthetic resin, in which ablank disc made of a synthetic resin material is integrally formed onthe outer periphery of a metal hub, i.e. a metal core, as a worm wheeland teeth are formed on the periphery of the blank disc by cutting andother means, to suppress the generation of uncomfortable sounds such asrattling sound and vibration sound.

For the material of the resin part of the above-described resin gear, MC(monomer-cast) nylon (trademark) not containing a reinforcing material,polyamide 6, polyamide 66, and polyamide 46 that are blended with areinforcing fiber material such as glass fiber, and the like are used inview of fatigue resistance, dimensional stability, and product cost.Generally, commercially available polyamide 6, polyamide 66, polyamide46, and the like contain glass fiber having a diameter of ca. 10 μm orca. 13 μm (refer to Japanese Examined Patent publication No. H06-60674(60674/1994)).

In a worm speed reduction mechanism used in an electric power steeringdevice, the worm is supported by two ball bearings, and a space betweenthese two ball bearings is filled with grease for lubrication betweenthe metal worm and the teeth of the worm wheel that is a resin gear.Generally, grease making use of mineral oil and poly-α-olefin oil inconsideration of thermal resistance is used as a base oil.

Further, a damper made of rubber that allows not only a pre-load to beapplied to the ball bearings arranged on both ends of the worm but also,when a small kick back is input from a tire side, information on thekickback to be transmitted only to a steering wheel by means of shiftingthe worm longitudinally so that a motor might not be influenced by arotation force is sometimes attached. Usually for the rubber material,acryl rubber represented by ethylene acryl rubber having a smallcompression set is most generally used.

However, electric power steering devices have recently come to be usedfor from a light car to a car carrying a class of engine having a pistondisplacement of 1,000 cc to 1,500 cc, and the electric power steeringdevices have been high-powered. In accordance with this, PV value, theproduct of the contact surface pressure P at the resin gear portion andthe peripheral velocity V, has become larger.

As a consequence of this, it has been found that the conventionally usedgrease that utilizes poly-α-olefin oil as abase oil is not effectiveenough for the lubrication condition between the driven gear (wormwheel) and the drive gear (worm). In addition, MC (monomer-cast) nylon(trademark) and generally commercially available polyamide resinscontaining glass fiber having a diameter of ca. 10 μm or ca. 13 μm havenot sufficed for wear resistance at a high PV value.

As the result, there has been a problem that temporary shortage of oilfilm occurs due to long operation of an electric power steering deviceat a high PV value and wearing of tooth flank of the gear graduallyprogresses, thereby increasing the risk of backlash at the meshingportions between the driven gear (worm wheel) and the drive gear (worm).Thus, it is expected that steering sense worsens and unusual noises(rattling sounds) occur. Further, there has also been a fear that thewhole electric power steering device becomes nonfunctional due todeformation of the gear, and in some cases, breakage of the gear.

In addition, there has been a possibility that the polyamide resinsdescribed above are highly water-absorptive in spite of being excellentin fatigue resistance and absorb water to swell the teeth portion of thegear of the worm wheel, spaces present between the worm and the wormwheel at an early stage after the production disappear, and the teethportion presses the worm by further swelling.

When the worm wheel swells as just described, steering becomes heavierbecause frictional resistance between the worm and the worm wheelbecomes larger, and the gear portion is worn out or broken due to anincrease of pressure on the gear portion and an increase of frictionalresistance, thereby giving rise to a failure of the electric powersteering device.

The object of the present invention is to solve the above problems andto provide an electric power steering device, in which grease containinga wax to prevent a gear portion from being worn out and broken as wellas to exert an efficient lubricating effect during a high temperatureoperation is used between a driven gear (worm wheel) and a drive gear(worm) and a resin gear suitable for power transmission of whichdimensional change due to water absorption is suppressed by making up aresin part with a polyamide resin containing glass fiber finer than oneconventionally used as a reinforcing fiber material and thus being freefrom fear of wear or breakage of the gear portion is used in the speedreduction gear mechanism constituting a power assist portion, and aresin gear used for the electric power steering device.

DISCLOSURE OF THE INVENTION

An electric power steering device according to the present inventionuses a speed reduction gear mechanism including a driven gear that is aresin gear and a drive gear that meshes with the driven gear. In theresin gear, a resin part formed with gear teeth on the outer peripheralsurface thereof is integrally formed on the periphery of a metal hub,and the resin part is formed of a resin composition having a polyamideresin, as a base resin, containing 10 to 50% by weight of glass fiberhaving a diameter of 5 to 9 μm.

Further, an adhesive layer composed of a silane coupling agent havingeither an epoxy group or an amino group at one end may be providedbetween the metal hub and the resin part. The glass fiber having adiameter of from 6 to 8 μm may be used, and the glass fiber having alength in the range of from 100 to 900 μm is used.

Still further, the resin part may be formed of a resin compositionhaving the polyamide resin, as a base resin, in which part of the glassfibers is substituted by carbon fibers.

Still further, grease applied to the meshed surfaces of the gears mayhave such a composition that includes a thickener and 3 to 10% by weightof a wax having a melting point or softening point in the range of 70 to130° C. in a base oil formed mainly of at least one oil selected frommineral oil, poly-α-olefin oil, and alkyl polyphenyl ether.

The resin gear can be applied to any of worm wheel, helical gear, spurgear, bevel gear, and hypoid gear.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front view explaining a structure of an electric powersteering device of the present invention;

FIG. 2 is a partial cross sectional view explaining a structure of aworm speed reduction gear mechanism of the electric power steeringdevice shown in FIG. 1;

FIG. 3 is a perspective view showing a structure of a worm and a wormwheel of the worm speed reduction gear mechanism;

FIG. 4 is a perspective view showing an appearance of a spur gear;

FIG. 5 is a perspective view showing an appearance of a helical gear;

FIG. 6 is a perspective view showing an appearance of a bevel gear;

FIG. 7 is a perspective view showing an appearance of a hypoid gear;

FIG. 8 is a representation explaining the kinds of additives added togrease and addition amounts thereof;

FIG. 9 is a representation explaining results of wear tests in a firsttest;

FIG. 10 is a representation explaining results of durability tests inthe first test;

FIG. 11 is a representation explaining test results of dimensionalstability and durability in a second test; and

FIG. 12 is a representation explaining results of wear tests in thesecond test.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention are explained.

[Structure of Electric Power Steering Device]

FIG. 1 is a front view explaining the structure of a column typeelectric power steering device 10 according to the present invention inwhich a resin gear suitable for power transmission according to thepresent invention is used in a speed reduction mechanism. In FIG. 1, thenumeral 11 represents a steering wheel shaft, 12 represents a steeringwheel shaft housing, 13 represents an electric motor, and 20 representsa rack-and-pinion motion conversion mechanism.

Although not shown in FIG. 1, the steering wheel shaft 11 is composed ofan upper steering wheel shaft 11 a and a lower steering wheel shaft 11b. The steering wheel shaft 11 is supported rotatably around the shaftcenter in the inside of the steering wheel shaft housing 12, and thesteering wheel shaft housing 12 is fixed at a predetermined locationwithin a vehicle compartment in a tilted state with the lower portiontoward the front. A steering wheel not illustrated is fixed to the upperend of the upper steering wheel shaft 11 a.

Further, the upper steering wheel shaft 11 a and the lower steeringwheel shaft 11 b are coupled by a torsion bar not illustrated, steeringtorque transmitted from the steering wheel via the upper steering wheelshaft 11 a to the lower steering wheel shaft 11 b is detected by thetorsion bar, and an output of the electric motor 13 is controlled basedon the detected steering torque.

The rack-and-pinion motion conversion mechanism 20 is placedapproximately horizontally in an engine room at the front of the vehiclesuch that its longitudinal direction is oriented to the right and leftdirection of the vehicle and is composed of a rack shaft 21 movable inthe shaft direction, a pinion shaft 22 including a pinion provided witha teeth portion that is supported obliquely to the shaft center of therack shaft 21 and meshes with a teeth portion of the rack shaft 21, anda tubular rack shaft case 23 that supports the rack shaft 21 and thepinion shaft 22.

The pinion shaft 22 and the lower portion of the lower steering wheelshaft 11 b are connected via universal couplings 25 and 26. The middleportion of the lower steering wheel shaft 11 b is arranged with a wormspeed reduction gear mechanism 30 described later and structured suchthat an auxiliary steering power is supplied from the electric motor 13to the lower steering wheel shaft 11 b.

FIG. 2 is a partial cross sectional view showing the structure of theworm speed reduction gear mechanism 30 of the electric power steeringdevice 10 described above, where the numeral 31 represents a worm wheel,32 represents a worm that meshes with the worm wheel 31, and 33 is agear case; The worm 32 is integrally formed with worm shafts 32 a and 32b at its both ends, and the worm shafts 32 a and 32 b are rotatablysupported by ball bearings 34 a and 34 b mounted respectively on thegear case 33. Further, the worm shaft 32 b is coupled to a driving shaft13 a of the electric motor 13 by spline or serration coupling.

The hub of the worm wheel 31, that is, a metal core 42 is coupled to thelower steering wheel shaft 11 b, and the rotation of the electric motor13 is transmitted to the lower steering wheel shaft 11 b via the worm 32and the worm wheel 31.

FIG. 3 is a perspective view showing the structure of the worm wheel 31and the worm 32 of the worm speed reduction gear mechanism 30 of theembodiment of the present invention. The worm wheel 31 is composed of ametal hub, that is, a metal core 42 worked appropriately by crossknurling and the like on the outer peripheral surface thereof and aresin part 43, fitted to the worked surface, that is formed of a resincomposition having a polyamide resin, as a base resin, containing 10 to50% by weight of glass fiber having a diameter of from 5 to 9 μm andformed with gear teeth 44 on the peripheral end surface thereof, wherethese are formed integrally with each other.

On the other hand, the worm 32 is made of metal, as is the case withconventional worms. The surface hardness of this worm 32 may be enhancedby subjecting to thermal treatment, nitriding, and the like as needed,thereby improving wear resistance to glass fiber contained in the wormwheel 31. Further, when sliding sounds generated by its sliding on theworm wheel 31 are considered, reduction in the sliding sounds can beachieved by adjusting surface roughness Ra to a range of from 0.02 to0.2 μm, more preferably from 0.02 to 0.06 μm. The surface roughness Ralower than 0.02 μm is difficult to work up and becomes costly, and thusnot practical. As a method for achieving the surface roughness Ra=0.02to 0.06 μm, a method of performing barrel finishing after polishing ispractical.

[Material for Worm Wheel]

The resin part 43 of the worm wheel 31 is preferably made of polyamide6, polyamide 66, or polyamide 46 that is excellent in fatigue resistanceas a base resin. The molecular weight of the polyamide resin is in therange capable of injection molding in a state containing glass fiber,specifically from 13,000 to 28,000 in the number average molecularweight, and more preferably in the rage of 18,000 to 26,000 in thenumber average molecular weight when fatigue resistance and moldabilityare considered. When the number average molecular weight is lower than13,000, fatigue resistance becomes deteriorated because the molecularweight is too low, and its practical utility is low. On the other hand,when the number average molecular weight exceeds 28,000, the meltviscosity becomes too high if glass fiber is contained in a practicalcontent of 15 to 35% by weight, and the production of a resin gear byinjection molding with high accuracy becomes difficult, which is notdesirable.

These base resins may be combined with other polyamide resins and resinssuch as polyolefin resin that is denatured by an acid anhydride toimprove wettability to a grease base oil consisting of a base oil of lowpolarity that is usually used between a worm and a worm wheel or withrubber-like materials such as ethylene-propylene-unconjugated diene(EPDM) rubber that improves shock resistance.

These base resins show durability higher than a certain level even bythemselves, work effectively against wearing out of the metal worm 32that is the counterpart material of the worm wheel 31, and functionadequately as a speed reduction gear. However, it is expected that thegear teeth 44 are worn out and damaged when used under harsherconditions, and therefore it is preferred to mix a reinforcing materialin order to enhance reliability.

As the reinforcing material, glass fiber having a diameter of from 5 to9 μm, more preferably in the range of from 6 to 8 μm is used andsurface-treated with a silane coupling agent having an epoxy group, anamino group, or the like on its one end in consideration of adhesivenessto the polyamide resin that is the base resin.

The silane coupling agent bound to the surface of the glass fiber actson amide bond of the polyamide resin through the functional group suchas epoxy group or amino group present on the one end and enhances thereinforcing effect of the glass fiber. At the same time, there is alsoan effect of suppressing dimensional change due to water absorption.

In other words, when glass fiber is contained in the polyamide resin atthe same weight content, the use of thinner glass fiber having adiameter of from 5 to 9 μm than a conventional one having a diameter of10 to 13 μm results in an increase of glass fiber pieces that act on theamide bond, thereby increasing mechanical strength such as tensilestrength and fatigue resistance such as flexural fatigue strength aswell as enhancing the effect of suppressing dimensional change due towater absorption.

However, when glass fiber having a diameter smaller than 5 μm is used,mechanical strength such as impact resistant strength tends to decreaseas well as increases the production cost, which lowers practical utilityand is undesirable.

The fiber length of the glass fiber is in the range of from 100 to 900μm, more preferably from 300 to 600 μm. When the fiber length is shorterthan 100 μm, it is too short to exert the reinforcing effect and theeffect of suppressing dimensional change due to water absorption, whichis not desirable. Further, when the fiber length exceeds 900 μm, fiberdamage during the process of molding the resin part and deterioration inmolding accuracy due to lowering of orientation are expected to occurdespite the fact that the reinforcing effect and the effect ofsuppressing dimensional change are enhanced, and thus molding of theresin part having teeth shape on the outer peripheral portion becomesdifficult, which is not desirable.

The content of glass fiber is from 10 to 50% by weight of the wholeresin, more preferably from 15 to 35% by weight. When the mixing ratioof the glass fiber is lower than 10% by weight, mechanical strength andthe effect of suppressing dimensional change due to water absorption arenot sufficient, which is not desirable. Further, when the mixing ratioof the glass fiber exceeds 50% by weight, there is a possibility thatthe worm 32 tends to be easily damaged and wearing away of the worm 32is promoted to result in lack of durability as a speed reduction gear,which is not desirable.

The glass fiber having a fiber diameter of from 5 to 9 μm explainedabove not only suppresses dimensional change due to water absorption butalso increases practical glass fiber pieces at the same mixing amount (%by weight) compared with conventional glass fiber having a fiberdiameter of from 10 to 13 μm. Therefore, it becomes possible to loadmore heavily by the increase of the glass fiber pieces, and the resinteeth portion becomes harder to be worn even when used under a highersurface pressure, thereby allowing adaptation to a use condition of ahigh surface pressure that is brought about by downsizing of a gear.

It should be noted that part of the glass fiber as the reinforcingmaterial may be substituted by a fibrous substance such as carbon fiber,or a whisker such as potassium titanate whisker and that a coloringagent and the like may also be added.

Further, it is desirable to add an iodide heat stabilizer and an amineantioxidant either alone or in combination to prevent deterioration byheat generated during molding and usage.

In order to further suppress a dimensional change of the polyamide resinthat is a base resin due to its water absorption, it is more effectiveto provide an adhesive layer between the outer periphery of the metalcore and the inner periphery of the resin part. To form the adhesivelayer, for example, there is a method in which the resin part issubjected to hot press fitting to the outer periphery of the metal coreafter applying a silane coupling agent thereon, followed byradiofrequency heating.

When the radiofrequency heating is carried out, not only is a strongadhesive layer formed but also the removal of residual stress generatedby press fitting can be carried out at the same time because only theinner periphery (interface) of the resin part adjacent to the outerperiphery of the metal core is melted. The adhesive force is enhancedwhen the temperature of the metal core portion is kept at from 200 to450° C. at the time of the radiofrequency heating. Although heating maybe carried out in an air atmosphere, oxidative deterioration of theresin and the like is suppressed by carrying out in an inert gasatmosphere such as argon gas, which is desirable.

The silane coupling agent used for adhesion has an alkoxyl group that isa hydrolyzable group on one end of its chemical structure, and thisalkoxy group is hydrolyzed to yield a hydroxyl group. This hydroxylgroup is condensed with a hydroxyl group on the metal surface bydehydration to form a covalent bond having a high bonding strength withthe metal. The other end of the silane coupling agent has an organicfunctional group, and this organic functional group forms a bond withamide bond in the molecular structure of the polyamide resin, therebygiving rise to strong binding between the metal core 42 and the resinpart 43.

As the organic functional group, amino group and epoxy group arepreferred. The silane coupling agent having such an organic functionalgroup includes

-   γ-glycidoxypropyltrimethoxysilane,-   β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,-   γ-aminopropyltriethoxysilane,-   N-β-(aminoethyl)-γ-aminopropyltriethoxysilane,-   γ-ureidopropyltriethoxysilane, and the like. Particularly, the    silane coupling agent having an epoxy group as the organic    functional group is high in reactivity with amide bond and more    desirable.

In order to allow the adhesive layer to be bound harder to the outerperiphery of the metal core, it is better to increase surface hydroxylgroups on the outer periphery of the metal core 42, and for thispurpose, surface processing by oxygen plasma and the like is furtherdesired.

With the aim of improving adherence between the metal core 42 and theresin part 43 and preventing the latter portion on the boundary with themetal core 42 from slipping away, inclusive of an increase of theadhesive force, shot blasting, knurling, and the like may be appliedbeforehand on the outer periphery of the metal core 42, andparticularly, knurling is desired. The depth of V-shaped groove inknurling is from 0.2 to 0.8 mm, and particularly preferably from 0.3 to0.7 mm.

[Grease]

Next, grease to keep excellent the lubrication between the teeth portionof the resin-made worm wheel and the teeth portion of the metal worm isexplained.

The grease used in the present invention is composed of a thickener anda base oil as the main components and further a wax that is added tokeep excellent the lubrication between the resin-made worm wheel and theworm under high surface pressure and has a melting point or softeningpoint in the range of 70 to 130° C. The base oil is at least oneselected from mineral oil, poly α-olefin oil, and alkyl polyphenylether. The thickener includes a urea compound made from amine andisocyanate, lithium soap, lithium complex soap, barium soap, bariumcomplex soap, and the like. Among these thickeners, a urea compoundhaving urea bond that is analogous to polyamide in structure isparticularly desirable because of excellent adsorption on the polyamideresin.

The wax having a melting point or softening point in the range of 70 to130° C. is solid at ordinary temperatures and dispersed in a state offine particles. When the temperature rises due to sliding contact of theresin worm wheel and the worm, the particles change to highly viscousliquid at their meshing portion. This highly viscous liquid raises theviscosity of a liquid itself (compatible liquid of base oil and wax) dueto its compatibility with a base oil that is lowered in viscosity by thetemperature rise, thereby preventing the liquid from escaping from themeshing portion as a whole. Even in a state that part of the wax remainswithout turning into the compatible liquid, the mixture stays at themeshing portion and keeps lubrication excellent even under a highsurface pressure.

As specific examples of these waxes, ester waxes such as montanoic acidester wax and partially saponified montanoic acid ester wax, hydrocarbonwaxes such as polyethylene wax, fatty acid amides such as stearoamide,lauryl amide, and behenic acid amide, and the like can be used. Amongthese waxes, hydrocarbon waxes having high compatibility with the baseoil described above are most preferred. The addition amount of thesewaxes is from 3 to 10% by weight, and more preferably from 4 to 8% byweight in a grease composition. When the addition amount is less than 3%by weight, the rise in viscosity due to the compatibility of the liquiditself at the time of operation is not enough, and improvement inlubrication is not sufficiently achieved, which is not desirable. Whenthe addition amount exceeds 10% by weight, the consistency of grease islowered, and the rise in viscosity at the time of turning into acompatible liquid becomes too high, thereby elevating a torque at thetime of gear operation and resulting in poor operability, which is notdesirable.

To improve wettability to the polyamide resin, diester oil, polyol esteroil, aromatic ester oil, and the like may be added to the base oil.Specifically, the diester oil includes dioctyl adipate (DOA), diisobutyladipate (DIBA), dibutyl adipate (DBA), dioctyl azelate (DOZ), dibutylsebacate (DBS) dioctyl sebacate (DOS), and the like.

The polyol ester oil includes pentaerythritol ester oil,dipentaerythritol ester oil, tripentaerythritol ester oil,neopentyl-type diol ester oil, and trimethylolpropane ester oil that arederivatized with C4 to C18 alkyl chain, and the like.

The aromatic ester oil includes trioctyl trimellitate (TOTM), tridecyltrimellitate, tetraoctyl pyromellitate, and the like.

These ester base oils are added to grease either alone or in combinationin the range of from 1 to 20% by weight, more preferably from 2 to 10%by weight relative to the total weight of the grease composition. Whenthe addition amount is less than 1% by weight, an effect of improvingwettability can not be obtained to a sufficient degree. On the otherhand, even when an amount exceeding 20% by weight is added, not only isa significant effect of improving wettability not obtained but also itis expected to have an adverse effect such as swelling on other partssuch as rubber damper to allow moving of the worm in the shaftdirection.

Furthermore, an oxidation stabilizer and other additives to improveantirust property and the like can be added to this grease. For example,an amine or phenol antioxidant, a rust inhibitor such as calciumsulfonate, an extreme-pressure additive such as MoDTC, and the like canbe listed.

As described, in the embodiment of the present invention, the worm speedreduction gear mechanism has been exemplified as a speed reduction gearmechanism of an electric power steering device. As a gear to be usedtherein, an example of the worm wheel that is a resin gear has beenexemplified, and the material of the resin gear, the grease to be used,and the like have been explained. However, the resin gear is not limitedto the worm wheel, and various modifications are possible. For example,the spur gear shown in FIG. 4, the helical gear shown in FIG. 5, thebevel gear shown in FIG. 6, the hypoid gear shown in FIG. 7, and thelike are possible as a gear shape.

[Tests for Dimensional Stability, Durability, and Wear Resistance ofWorm Wheel and their Evaluation Results]

Next, tests for dimensional stability, durability, and wear resistanceof worm wheel and their evaluation results are explained.

A. A First Test and its Evaluation Results

In a first test, tests for durability and wear resistance were carriedout with respect to a plurality of examples of worm wheels andcomparative examples and evaluated. It should be noted that the presentinvention is not limited at all by the examples and the comparativeexamples explained below.

a. Resin Gear Part

(1) Structure 1

Metal core: Steel (material JIS-S45C) having an outer diameter of 65 mmand a width of 16 mm with knurled grooves of a depth of 0.5 mm.

Resin: Polyamide 6 (containing 30% by weight of glass fiber (GF) havinga diameter of 6 μm, UBE nylon (trademark) produced by Ube Industries,Ltd., a copper iodide heat stabilizer contained). The glass fiber havinga diameter of 6 μm means glass fiber having an average diameter ofapproximately 6 μm and contains glass fibers having diameters in therange of from 5 to 7 μm.

Molding of resin part: Insert molding using the metal core as a core.

Peripheral shape of resin part at the time of molding: Inner diameter of64 mm, outer diameter of 83 mm, and width of 15.5 mm having a helicalshape with a cutting allowance left.

After molding, teeth of the resin part were further cut to finishfinally in a worm wheel shape.

(2) Structure 2

Glass fiber having a diameter of 8 μm was used as the glass fiber (GF)mixed with the resin, but otherwise the structure was the same as in thestructure 1. Here, the glass fiber having a diameter of 8 μm means glassfiber having an average diameter of approximately 8 μm and containsglass fibers having diameters in the range of from 7 to 9 μm.

(3) Structure 3

Metal core: Steel (material JIS-S45C) having an outer diameter of 65 mmand a width of 16 mm with knurled grooves of a depth of 0.5 mm. Afterdegreasing, soaking in a methanol solution of 10% by weight ofγ-glycidoxypropyltrimethoxysilane (“A-187” produced by NihonunicaCorporation) that is a silane coupling agent having an epoxy group, andthen drying in the air with the aim of forming an adhesive layer, acoating of the silane coupling agent was formed on the surface of themetal core.

Resin: Polyamide 6 (containing 30% by weight of glass fiber (GF) havinga diameter of 6 μm, UBE nylon (trademark) produced by Ube Industries,Ltd., a copper iodide heat stabilizer contained).

Molding of resin part: Molding as a body separated from the metal core.

Peripheral shape of resin part at the time of molding: Inner diameter of64 mm, outer diameter of 83 mm, and width of 15.5 mm having a helicalshape with a cutting allowance left.

High frequency fusion: The resin part treated against water absorptionwas heated for 20 min at 140° C. to swell the resin part, and thenpress-fitted to the metal core. Subsequently, the resin part was fused(adhered) to the metal core by high frequency heating under an argonatmosphere until the temperature of the metal core rose to 350° C.,followed by rapid cooling by dipping in water. Then, teeth of the resinpart were cut to finish finally in a worm wheel shape.

(4) Structure 4 (Comparative Example 1)

Although the structure was almost the same as in the structure 1, glassfiber having a diameter of 10 μm was used as the glass fiber (GF) mixedwith the resin. Here, the glass fiber having a diameter of 10 μm meansglass fiber having an average diameter of approximately 10 μm andcontains glass fibers having diameters in the range of from 9 to 11 μm.

(5) Structure 5 (Comparative Example 2)

Although the structure was almost the same as in the structure 1, glassfiber having a diameter of 13 μm was used as the glass fiber (GF) mixedwith the resin. Here, the glass fiber having a diameter of 13 μm meansglass fiber having an average diameter of approximately 13 μm andcontains glass fibers having diameters in the range of from 12 to 14 μm.

b. Preparation of Grease

FIG. 8 is a representation explaining the kinds of additives added togrease and their addition amounts. A base grease in which poly-α-olefinoil (8 mm²/s at 100° C.) was used as the base oil for grease and analiphatic diurea compound was used as a thickener (content of thickener:13% by weight) was added with various additives (wax, antioxidant,antirust) shown in FIG. 8 to prepare three kinds of grease consisting ofcomposition A, composition B, and composition C having consistency No.2.

Wax: Polyethylene wax (Mitsui Hiwax 320P (Molecular weight 3,000,softening point 114° C.) produced by Mitsui Chemicals, Inc.)

Antioxidant: 4,4′-Dioctyldiphenylamine (Nonflex OD-RH produced by SeikoChemical Co., Ltd.)

Antirust: Neutral calcium sulfonate (Molesco-Amber SC45N (mineral oilcontent 54%) produced by Matsumura Oil Research Corp.)

c. Combination of Resin Gear and Grease in Test Apparatus

The above-described structures 1 to 5 for resin gear and theabove-described grease, Composition A, Composition B, and Composition C,were set in an apparatus by the following combinations.

EXAMPLE 1 Resin gear: Structure 1 Grease: Composition A EXAMPLE 2 Resingear: Structure 2 Grease: Composition A EXAMPLE 3 Resin gear: Structure3 Grease: Composition A COMPARATIVE EXAMPLE 1 Resin gear: Structure 4Grease: Composition A COMPARATIVE EXAMPLE 2 Resin gear: Structure 5Grease: Composition A COMPARATIVE EXAMPLE 3 Resin gear: Structure 1Grease: Composition B COMPARATIVE EXAMPLE 4 Resin gear: Structure 1Grease: Composition C COMPARATIVE EXAMPLE 5 Resin gear: Structure 5Grease: Composition B

d. Test Method and Evaluation Results of Wear Resistance

In order to simulate lubrication between the worm (made of metal) andthe worm wheel (resin gear) of an actual electric power steering device,plate test pieces were prepared by using the same materials as the resinmaterials used in the examples 1 and 2 and the comparative examples 1 to5, and these test pieces were allowed to come in contact with revolvingsteel balls (peripheral speed of 1 m/sec at the contact portion of theball top) of a ball-on-disk test apparatus in which three steel balls(SUJ2) having a diameter of 6.35 mm are arranged 120 degrees apart, andwear resistance was evaluated. At an atmospheric temperature set to 80°C. and in a state that grease (Composition A, Composition B, orComposition C) was present between the test piece and the balls,rotation was continued for 8 hours under each load increased beginningfrom 2 kg and in increments of 0.5 kg, and then signs of wear of thetest piece were observed. The results of the wear test are shown in FIG.9.

As is apparent from FIG. 9, when a resin gear mixed with glass fiberthinner than conventional glass fiber as the reinforcing material andgrease containing a prescribed amount of wax were combined, wearoccurrence was not observed even after using under a high load andpractically high surface pressure, and wear resistance was found to beimproved.

e. Test Method and Evaluation Results of Durability

In the test of durability, the gears having the combinations shown inthe above examples 1 to 3 and the comparative examples 1 to 5 wereinstalled on an actual electric power steering device, respectively;grease was filled (grease was evenly applied to the outer peripheralsurface of the resin part of the worm wheel and the surface of theworm); in an environmental condition 1, the atmospheric temperature andthe relative humidity were set to 80° C. and 30%, respectively; in anenvironmental condition 2, the atmospheric temperature and the relativehumidity were set to 80° C. and 70%, respectively; steering wasperformed 100,000 times; and the amount of wear of the worm wheel notlarger than 40 μm from the initial value was considered to beacceptable. The amount of wear was measured every 10,000 times ofsteering. Further, a case in which the space between the resin gear andthe worm was decreased due to a dimensional change caused by waterabsorption and the operability (operation torque) was worsened more than20% was considered to be unacceptable. The results of the durabilitytest are shown in FIG. 10.

In the environmental condition 1, the tendency of the maximum load atwhich wear did not occur as shown in FIG. 9 and the tendency of thedurable number at which the amount of wear exceeded the criterion werein approximate agreement. In the environmental condition 2 where aninfluence by the dimensional change due to water absorption becamelarger, only the example 3 in which adhesion by high frequency fusionwas combined could endure 100,000 times of steering and was judged to beacceptable. In the examples 1 and 2, the amount of wear did not exceedthe criterion, but the operability deteriorated due to dimensionalchange caused by water absorption, thereby having been judged to beunacceptable.

B. A Second Test and its Evaluation Results

In a second test, tests for dimensional stability, durability, and wearresistance were carried out with respect to a plurality of examples andcomparative examples of worm wheels shown below and evaluated. It shouldbe noted that the present invention is not limited at all by thefollowing examples and the comparative examples.

a. Structures of Examples and Comparative Examples

EXAMPLE 11

Metal core: Steel (material JIS-S45C) having an outer diameter of 65 mmand a width of 16 mm with knurled grooves of a depth of 0.5 mm.

Resin: Polyamide 6 containing 30% by weight of glass fiber (GF) having adiameter of 6 μm (UBE nylon (trademark) produced by Ube Industries,Ltd., a copper iodide heat stabilizer contained).

The glass fiber having a diameter of 6 μm means glass fiber having anaverage diameter of approximately 6 μm and contains glass fibers havingdiameters in the range of from 5 to 7 μm.

Molding of resin part: Insert molding using the metal core as a core.

Peripheral shape of resin part at the time of molding: Inner diameter of64 mm, outer diameter of 83 mm, and width of 15.5 mm having a helicalshape with a cutting allowance left.

After molding, teeth of the resin part were further cut to finishfinally in a worm wheel shape.

EXAMPLE 12

Glass fiber having a diameter of 7 μm was used as the glass fiber (GF)mixed with the resin, but otherwise the same as in the example 11. Here,the glass fiber having a diameter of 7 μm means glass fiber having anaverage diameter of approximately 7 μm and contains glass fibers havingdiameters in the range of from 6 to 8 μm.

EXAMPLE 13

Metal core: Steel (material JIS-S45C) having an outer diameter of 65 mmand a width of 16 mm with knurled grooves of a depth of 0.5 mm. Afterdegreasing, soaking in a methanol solution of 10% by weight ofγ-glycidoxypropyltrimethoxysilane (“A-187” produced by NihonunicaCorporation) that is a silane coupling agent having an epoxy group, andthen drying in the air with the aim of forming an adhesive layer, acoating of the silane coupling agent was formed on the surface of themetal core.

Resin: Polyamide 6 containing 30% by weight of glass fiber (GF) having adiameter of 6 μm (UBE nylon (trademark) produced by Ube Industries,Ltd., a copper iodide heat stabilizer contained).

The glass fiber having a diameter of 6 μm means glass fiber having anaverage diameter of approximately 6 μm and contains glass fibers havingdiameters in the range of from 5 to 7 μm.

Molding of resin part: Molding as a body separated from the metal core.

Peripheral shape of resin part at the time of molding: Inner diameter of64 mm, outer diameter of 83 mm, and width of 15.5 mm having a helicalshape with a cutting allowance left.

High frequency fusion: The resin part treated against water absorptionwas heated for 20 min at 140° C. to swell the resin part, and thenpress-fitted to the metal core. Subsequently, the resin part was fused(adhered) to the metal core by high frequency heating in an argonatmosphere until the temperature of the metal core rose to 350° C.,followed by rapid cooling by dipping in water. Then, teeth were formedby cutting the resin part to finish finally in a worm wheel shape.

COMPARATIVE EXAMPLE 11

Although almost the same as in the example 11, glass fiber having adiameter of 10 μm was used as the glass fiber (GF) mixed with the resin.Here, the glass fiber having a diameter of 10 μm means glass fiberhaving an average diameter of approximately 10 μm and contains glassfibers having diameters in the range of from 9 to 11 μm.

COMPARATIVE EXAMPLE 12

Although almost the same as in the example 11, glass fiber having adiameter of 13 μm was used as the glass fiber (GF) mixed with the resin.Here, the glass fiber having a diameter of 13 μm means glass fiberhaving an average diameter of approximately 13 μm and contains glassfibers having diameters in the range of from 12 to 14 μm.

b. Tests for Dimensional Stability, Durability, and Wear Resistance, andtheir Evaluation Results

FIG. 11 represents the test results of dimensional stability anddurability, and FIG. 12 represents the test result of wear resistance.First, their test methods are explained.

b-1. Test for Dimensional Stability

In the test for dimensional stability, a dimensional change of the outerdiameter of a gear was measured for each of the above examples 11 to 13and the comparative examples 11 and 12 after left standing for 70, 300,and 500 hours, respectively, under the following environmentalconditions A and B. The test result showing a change not larger than 40μm under any of the conditions was considered to be acceptable andexpressed as “O”, while the test result showing a variation exceeding 40μm was considered to be unacceptable and expressed as “X”.

Condition A: Temperature 60° C., relative humidity 90%

Condition B: Temperature 80° C., relative humidity 90%

b-2. Test for Durability

In the test of durability, the worm wheels of the above examples 11 to13 and the comparative examples 11 and 12 were installed on an actualelectric power steering device, respectively, and steering operationswere repeated under the following environmental conditions C, D, E, andF to test durability. The test result was considered to be acceptableand expressed as “O” when a gear could tolerate 100,000 times ofsteering operations under any of the conditions, while the test resultwas considered to be unacceptable and expressed as “X” when a gear couldnot tolerate 100,000 times of steering operations.

-   Condition C: Temperature 30° C., relative humidity 50%-   Condition D: Temperature 50° C., relative humidity 90%-   Condition E: Temperature 80° C., relative humidity 50%-   Condition F: Temperature 80° C., relative humidity 90%

As is apparent from the test results of the dimensional stability andthe durability shown in FIG. 11, it was confirmed that dimensionalchange due to water absorption by the resin was suppressed by usingglass fiber having a diameter of 6 to 7 μm as the glass fiber (GF) addedto the resin. In conjunction with that, durability was found to beexcellent under harsh environments of high temperature and highhumidity.

b-3. Test for Wear Resistance

The test for wear resistance was carried out by forming plate testpieces with the use of the same resin materials as the resin materialsof the worm wheels used in the examples 11 and 12 and the comparativeexamples 11 and 12, and subjecting to the ball-on-disk test using thesetest pieces and SUJ-made balls (three balls were arranged equally 120degrees apart, the test piece was rotated, the peripheral speed at thecontact portion of the ball top was 1 m/sec).

At a pressure on the contact surface Pmax set in the range of from 150to 200 megapascals (MPa) and an atmospheric temperature set to 80° C.and in a state that grease (base oil: poly-α-olefin oil, thickener:aliphatic urea) was present between the test piece and the ball,rotation was continued for 8 hours, and then signs of wear of the testpiece were observed.

As is apparent from the wear test results shown in FIG. 12, it was foundthat wear did not occur and wear resistance was excellent by the use ofglass fiber having a diameter of 6 to 7 μm as the glass fiber (GF) mixedwith the resin even when used under the conditions of high contactsurface pressure.

Although glass fibers having diameters of 6 to 7 were used as the glassfiber (GF) in the examples 11 to 13 explained above, it was confirmedthat approximately the same results were obtained even when glass fiberhaving a diameter of 5 to 9 μm was used, showing excellent dimensionalstability, durability, and wear resistance. Therefore, in the presentinvention, glass fiber having a diameter of 5 to 9 μm is most suitablein size as the glass fiber (GF) mixed with the resin forming the resingear.

The resin gear used in speed reduction gear mechanism for electric powersteering device explained in the embodiments described above can beapplied not only to the speed reduction gear mechanism for electricpower steering device but needless to say to gear mechanism in generalregardless of its use.

INDUSTRIAL APPLICABILITY

The electric power steering device and the resin gear used therein ofthe present invention can be an electric power steering device havinghigh reliability and excellent durability in which generation ofuncomfortable sounds such as rattling sound and vibration sound issuppressed when used for an electric power steering device for vehicle.Further, the resin gear can also be applied to gear mechanism in generalin addition to the electric power steering device.

1. An electric power steering device to transmit an auxiliary power byan electric motor to a steering mechanism of a vehicle via a speedreduction gear mechanism, the speed reduction gear mechanism comprising:a driven gear, that is a resin gear, having a resin part with gear teethformed on an outer peripheral surface thereof and integrally formed onthe outside of a metal core; a drive gear that meshes with the drivengear; and grease present at least between the driven gear and the drivegear, wherein the resin part of the resin gear is composed of a resincomposition having a polyamide resin, as a base resin, containing 10 to50% by weight of glass fiber having a diameter in the range of from 5 to9 μm, and the grease is composed of a base oil formed mainly of at leastone oil selected from mineral oil, poly α-olefin oil, and alkylpolyphenyl ether, a thickener, and 3 to 10% by weight of a wax having amelting point or softening point in the range of from 70 to 130° C. 2.The electric power steering device according to claim 1, wherein anadhesive layer composed of a silane coupling agent having either one ofepoxy group or amino group on one end thereof is provided between themetal core and the resin part of the resin gear.
 3. The electric powersteering device according to claim 1, wherein the diameter of the glassfiber contained in the resin part of the resin gear is in the range offrom 6 to 8 μm.
 4. The electric power steering device according to claim1, wherein the length of the glass fiber contained in the resin part ofthe resin gear is in the range of from 100 to 900 μm.
 5. The electricpower steering device according to claim 1, wherein the resin part ofthe resin gear is composed of a resin composition having the polyamideresin, as the base resin, with the glass fiber partially substituted bycarbon fiber.
 6. The electric power steering device according to claim1, wherein the driven gear and the drive gear are in a form of wormwheel, helical gear, spur gear, bevel gear, or hypoid gear.
 7. A resingear suitable for power transmission, the resin gear having a resin partwith gear teeth formed on an outer peripheral surface thereof andintegrally formed on the outside of a metal core, the resin part beingcomposed of a resin composition having a polyamide resin, as a baseresin, containing 10 to 50% by weight of glass fiber having a diameterin the range of from 5 to 9 μm.
 8. The resin gear according to claim 7,wherein an adhesive layer composed of a silane coupling agent havingeither one of epoxy group or amino group on one end thereof is providedbetween the metal core and the resin part.
 9. The resin gear accordingto claim 7, wherein the diameter of the glass fiber contained in theresin part is in the range of from 6 to 8 μm.
 10. The resin gearaccording to claim 7, wherein the length of the glass fiber contained inthe resin part is in the range of from 100 to 900 μm.
 11. The resin gearaccording to claim 7, wherein the resin part is composed of a resincomposition having the polyamide resin, as the base resin, with theglass fiber partially substituted by carbon fiber.
 12. The resin gearaccording to claim 7, wherein the resin gear is a worm wheel, a helicalgear, a spur gear, a bevel gear, or a hypoid gear.